1. Field of the Invention
The invention relates generally to the production of film coatings and more specifically to the production of film coatings of sorbent materials by matrix assisted pulsed laser deposition.
2. Description of the Related Art
Chemical sensors commonly use coatings of chemoselective materials to effect the detection of chemical analytes. Chemoselective materials are substances that are chosen for their ability to interact with specific chemical analytes. A typical chemical sensing device includes a substrate transducer, a thin film coating of a chemoselective material on the substrate and a means for detecting the interaction of the chemoselective material with a chemical analyte. In a surface acoustic wave (SAW) device, the substrate is typically a piezoelectric material that is used to propagate a surface acoustic wave between sets of interdigitated electrodes. In a SAW chemical sensor, the chemoselective material is coated on the surface of the transducer. When a chemical analyte interacts with a chemoselective material coated on the substrate, the interaction results in a change in a physical characteristic of the coating which results in a detectable change in the SAW properties such as the amplitude or velocity of the propagated wave. The detectable changes in the characteristics of the wave indicates the presence of the chemical analyte. SAW devices are described in numerous patents and publications including U.S. Pat. No. 4,312,228 to Wohltjen and U.S. Pat. No. 4,895,017 to Pyke, the disclosures of which are hereby incorporated by reference. Other types of chemical sensors known in the art that use chemoselective coatings include bulk acoustic wave (BAW) devices, plate acoustic wave devices, interdigitated microelectrode (IME) devices, and optical waveguide (OW) devices, electrochemical sensors, and electrically conducting sensors.
The operating performance of a chemical sensor that uses a chemoselective film coating is greatly affected by the thickness, uniformity and composition of the coating. For some chemical sensor technologies, the sensitivity of the sensor to a chemical analyte increases with increasingly thicker coatings. However, for some types of sensors, increasing the coating thickness has a detrimental effect on the sensitivity. In these types of sensors, only the portion of the coating immediately adjacent to the transducer substrate is sensed by the transducer. If the coating thickness is too thick, the outer layers of the coating material, that is, the layers farthest away from the substrate, are not sensed. These outer layers of coating material compete for the analyte with the layers of coating being sensed and thus reduce the sensitivity of the chemical sensor. Further, as the thickness of the chemoselective coating is increased, the time taken for an analyte to diffuse into the coating and come to thermodynamic equilibrium is increased and hence the time taken to reach an equilibrium sensing signal is increased. Thus, the thickness of the coating is a critical factor in the performance of real time monitoring chemical sensors, affecting the response time, recovery time and response magnitude of the sensor. Uniformity of the coating is also a critical factor in the performance of a sensor that uses a chemoselective coating. In this regard, it is important not only that the coating be uniform and reproducible from one device to another, so that a set of devices will all operate with the same sensitivity, but also that the coating on a single device be uniform across the active area of the substrate. If a coating is non-uniform, the response time to analyte exposure and the recovery time after analyte exposure are increased and the operating performance of the sensor is impaired. The thin areas of the coating respond more rapidly to an analyte than the thick areas. As a result, the sensor response signal takes longer to reach an equilibrium value, and the results are less accurate than they would be with a uniform coating. Further, in a chemical sensing device that uses acoustic wave energy in the detection of interactions between a chemoselective coating and analyte molecules, a non-uniform coating causes a greater amount of insertion loss of the acoustic signal than does a uniform coating. Insertion loss is caused by the loss of wave energy to the coating, for example, in the form of heat. This loss is exacerbated by irregularities of the coating surface. If insertion loss can be reduced by providing a more uniform coating, it would be possible then to increase the thickness of the coating without significantly impairing the operational ability of the device. Further, by reducing the insertion loss by producing a more uniform coating, a device may functionally operate with a larger dynamic operating range. An additional advantage to having a more uniform coating is that the coating is less likely than a nonuniform coating to delaminate from the substrate surface.
Conventional methods to produce film coatings on substrates or chemical sensing devices involve dissolving the coating material in a volatile solvent and applying the solution to the substrate surface by pipetting or spray coating. The substrate surface may be rotated at high speed in a technique called spin coating. These techniques have several disadvantages. It is difficult with the spin coating or spray coating methods to control the coating thickness precisely, or to ensure that the coating is uniform from one batch to another. Spray coating provides no control over the uniformity of a coating over a substrate surface. Spin coating potentially provides a more uniform coating surface than does spray coating, but nevertheless this method has the disadvantage that the edges of the coating tend to be thicker than the interior. If a plurality of devices on a single substrate are coated in a single batch, the devices closer to the outer edge of the substrate will have a thicker coating than the devices closer to the center of the substrate. Further, the spin coating method is difficult to scale up. The spin coating method is also awkward, unwieldy and wasteful for coating large surfaces at one time because of the difficulty of spinning a large substrate and because of the loss of material off the edges of the substrate during the spinning process. Also, the spin coating method is poorly suited for coating discrete areas of a substrate while leaving other areas uncoated, as might be desired when, for example, several devices are to be coated in a single batch or when only the active area of a device is to be coated. Leaving an area of a substrate uncoated in a spin coating process requires the use of tape, which can introduce impurities on a substrate. Moreover, the spray coating and spin coating methods are not useful to create coatings of materials that cannot dissolved in a solvent and are poorly suited for creating multilayer coatings.
Thermal evaporation under a vacuum is another method of creating a film coating. This method is usable only for compounds that do not decompose at the required operating temperature.
More precise and accurate control over the thickness and uniformity of a film coating may be achieved by using pulsed laser deposition (PLD), a physical vapor deposition technique that has been developed recently for forming ceramic coatings on substrates. By this method, a target comprising the stoichiometric chemical composition of the material to be used for the coating is ablated by means of a pulsed laser, forming a plume of ablated material that becomes deposited on the substrate. Although the method is used primarily to create coatings of oxide ceramics such as ferroelectrics, ferrites and high T.sub.c superconductors, it has also been used to create organic or polymer coatings for various uses. U.S. Pat. No. 4,604,294 to Tanaka et al, U.S. Pat. No. 5,192,580 to Blanchet-Fincher and U.S. Pat. No. 5,288,528 to Blanchet-Fincher disclose methods of making organic or polymeric thin films by laser vapor-deposition. In these methods, certain bonds of the organic compound or polymer are photochemically broken, releasing low molecular weight fragments that condense and repolymerize on a substrate. Similar methods are also discussed in Ogale, S. B., "Deposition of Polymer Thin Films by Laser Ablation", in Pulsed Laser Deposition of Thin Films, Chrisey, D. B. and Hubler, G. K., Eds. John Wiley & Sons, New York, 1994, Chapter 25; Hansen S. G. and Robitaille, T. E., "Formation of Polymer Films by Pulsed Laser Evaporation" Appl. Phys. Lett. 52 (1), Jan. 4, 1988, 81-83; Kale et al. "Deposition of Amorphous Fluoropolymers Thin Films by Laser Ablation" Appl. Phys Lett. 62 (5), Feb. 1, 1993, 479-481; Kale et al, "Deposition of Polyphenylene Sulphide (PPS) Polymer by Pulsed Excimer Laser Ablation", Materials Letters 15 (1992) 260-263; and Kale et al "Degradation of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x Thin Epitaxial Films in Aqueous Medium and Control of Degradation Using Polymer Overlayers Deposited by Pulsed Excimer Laser" Thin Solid Films 206 (1991) 161-164. A method of producing collagen thin films by laser deposition is disclosed in the commonly assigned U.S. patent application Ser. No. 08/655,788. The disclosures of the above patents, patent applications and publications are incorporated herein by reference.
Another factor that affects sensor performance is the chemical composition of the film coating. For some applications, it is desirable that a film coating be created with a minimum of fragmentation, rearrangement, degradation or damage to the material being transferred. This is particularly true in the creation of chemoselective films for chemical sensing devices, since chemical selectivity for a particular analyte often depends on the precise arrangement of substituents on the chemoselective material. A drawback to using conventional pulsed laser deposition in the creation of film coatings is that direct ablation of the target can be stressful and damaging to fragile materials. Chemoselective polymers used as coatings in chemical sensing devices commonly contain sensitive functional groups that can be easily destroyed by bond scission processes or other unwanted reactions if they are exposed to too much energy or stress.
Methods of ablating and ionizing large molecules for mass spectral analysis have been described. U.S. Pat. No. 4,920,264 to Becker, the disclosure of which is incorporated herein by reference, describes a method of desorbing large, nonvolatile, thermally labile molecules from a substrate by laser ablation by combining the large molecule with a solvent and freezing the mixture and then exposing the frozen mixture to laser radiation. The desorbed molecules are ionized and introduced into a mass analysis zone or are introduced into a liquid chromatography interface. U.S. Pat. No. 5,118,937 to Hillencamp et al, the disclosure of which is incorporated herein by reference, describes a method of laser desorption and ionization of large biomolecules by combining the biomolecules with a matrix that absorbs laser light at a wavelength of 300 nm or greater and irradiating the specimen with laser light in the range absorbed by the matrix. The desorbed biomolecules are then ionized and introduced into a mass analyzer. U.S. Pat. No. 5,135,870 to Williams et al, the disclosure of which is incorporated herein by reference, describes a method of pulsed laser ablation and ionization of high molecular weight compounds by combining the compounds with a solvent, freezing the solution, creating a thin film of the frozen solution on a sample stage, and irradiating the sample stage to create a plume containing the high molecular weight compound.